Use microphone change preferences firefox / opera is different

Hi, when wanting to adapt the preferences for audio/video I cannot select in firefox the dropdown of the microphone. Strange is when I start a meeting in opera, I can change it, but the issue there is that the view of the meeting video is suddenly mirroring and duplicating itself endlessly.

  1. what do to do in opera to avoid that ?
  2. enable in firefox that it uses different audio ?
    I have a webcam and a realtek speaker (I would use the microphone of the webcam to do the talking, and use the realtek speaker microphone to share a video on my desktop / and share music of my mp3 player (vlc / SMplayer))
    Thanks